Introduction
Species of Sorbus L. are small to medium sized trees abundant in coastal areas, with three European hotspots for continued Sorbusdiversification and speciation: Fennoscandia, South-East Europe, and Britain (Robertson et al., 2010).
Sorbus is an agamic species complex, resulting from hybridization, polyploidy and apomixis (Hörandl, 1998), where interspecific hybridization, the main resource of diversity inSorbus , is usually followed by apomictic reproduction. Due to rapid evolution, speciation, and hybridization, multiple speciation events have occurred in parallel within different lineages ofSorbus (Li et al., 2017). The study of Sorbus faces several complex taxonomic issues that have yet to be resolved in Europe (Sennikov and Kurtto, 2017). Hybridization between species with differing chromosome numbers followed by polyploidization and the production of seed sterile offspring are propagated and stabilized as new taxa by apomixis.
The genus Sorbus belongs to the apple tribe Maleae in the Rosaceae family and in the traditional sense used to include more than 100 species widespread in the Northern Hemisphere according to POWO (2022). Hybridogenous taxa of Sorbus have been commonly left unidentified, until Sennikov and Kurtto (2017) assigned a new intrageneric nomenclature for the nothotaxa. Hybridogenous species are mostly regarded as species in the same way as non-hybrid species, whereas causal hybrids are not equivalent to species (Baker and Bradley, 2006). Recently, several new genera were established for all hybrid taxa between Aria , Sorbus , Torminalis , andChamaemespilus by Sennikov and Kurtto (2017). The majority of the Norwegian species are hybrids between Aria and Sorbus , hence treated under the new genus name Hedlundia . This approach is still under debate (Levin et al., 2018), but obviously Sorbusis non-monophyletic in its traditional large circumscription, as evidence in later years has shown that Sorbus has five main evolutionary branches and is polyphyletic (Lo and Donoghue, 2012; Tang et al., 2022).
Currently, 13 species of Sorbus in a wide sense includingSorbus hybrids are registered in Norway. Some of these taxa are very common, and others are endemic and listed as threatened species (IUCN, 2022). Taxonomical challenges within Sorbus have been discussed amongst Nordic botanists for decades, starting with Hedlund (1901) publishing the first comprehensive monograph describing the variation in Sorbus species. Later Liljefors (1955) hypothesized on how the Nordic taxa of Sorbus were related, and how the sexual diploids at the base of the lineages Sorbus , Aria ,Torminalis , and Chamaemespilus have been hybridizing, resulting in triploid and tetraploid taxa, forming the backbone of the origin of the polyploid endemics.
The base chromosome number of Sorbus in a wide sense is x=17. Diploid, triploid, and tetraploid members are respectively 2n=2x=34, 2n=3x=51 and 2n=4x=68 (Pellicer et al., 2012).
The five hybridogenous triploid species of Hedlundia (H. subarranensis , H. sognensis , H. subpinnata , H. neglecta , and H. lancifolia ) have chromosome numbers 2n=51, with semipinnated simple leaves (Sennikov et al., 2016). H. subarranensis has a very limited distribution in western Norway, endemic to Norway, and is listed as endangered by IUCN. H. sognensis is even more rare, endemic to also western Norway, and listed as endangered, as H. subpinnata , endemic and limited to south-east Norway. H. neglecta and H. lancifolia are found in northern Norway and are both endemic. H. neglecta is listed as endangered by IUCN, while H. lancifolia is critically endangered.
Tetraploid hybridogenous species with 2n=4x=68 chromosomes of this complex include H. meinichii , H. subsimilis , H. hybrida , H. mougeotii , A. obtusifolia and A. rupicola . The four species H. subsimilis , H. mougeotii ,A. obtusifolia and A. rupicola have simple leaves, whileH. meinichii and H. hybrida have compound leaves. H. meinichii grows in coastal areas in southern Norway and is endemic to Norway. H. meinichii is very variable, including both triploids and tetraploids, and several different populations containing distinct morphs (Salvesen, 2011). It is listed as vulnerable by IUCN but is unsolved due to undescribed variation. H. subsimilis is distributed in southern parts of Norway, is endemic to this area, and listed as vulnerable by IUCN. H. hybrida is common along the coast in Norway and is as H. meinichii a variable species with several morphs. H. mougeotii is widespread in Europe, but scattered in Norway, as it is an introduced species. A. obtusifolia is distributed along the coast of southern Norway with a few finds just across the Swedish border and is endemic to Scandinavia.A. rupicola has a wider distribution in northwest Europe and is found scattered along the coast to northern Norway.
Morphological traits of leaves and fruits have for a long time been used to classify hybridogenous Sorbus taxa (Hedlund, 1901). The shape of leaves has proven useful to differentiate these taxa to genus and species level. On the extremes, Sorbus aucuparia (2n) has compound leaves divided into 5-8 pairs of serrated leaflets and is distributed all over Norway as a very common species, while Aria edulis (2n), Aria rupicola (4n), and Aria obtusifolia(4n) have simple leaves and a limited distribution in Norway, the former only as naturalized from gardens. All species of Hedlundiapresented here have leaves of a shape intermediate between these opposite types.
Sorbus pollen morphology is diverse, and there is significant variation at the intra and interspecies level (Boyd and Dickson, 1987). Using only a light microscope (LM) the separation of pollen from species within the genus is difficult, as Sorbus pollen is very similar to other taxa of the Rosaceae family (Boyd and Dickson, 1987; Eide, 1981; Reitsma, 1966). Most studies of fossil pollen have not identifiedSorbus pollen to lower taxonomic levels than Sorbus -type or Rosaceae. Sorbus pollen is tricolporate, psilate or intrastriate, and suboblate to subprolate with maximum length of 23-42 µm (Boyd and Dickson, 1987). Gabrielian (1978) separated diploid species of Sorbus from Western Asia and the Himalayas based on shape and size of pollen, type and number of apertures, and thickness and structure of the exine. Using scanning electron microscope (SEM), identifications can be done to a lower taxonomic level than if you use light microscope (LM), as Bednorz et al. (2005) has shown to study the Polish species of Sorbus . Bednorz et al. (2005) describesSorbus pollen as belonging to the non-operculate group as it lacks an aperture membrane, and the differences in costa colpi are the most important feature. Despite few studies on the pollen morphology ofSorbus species, little is known in general about the Norwegian hybridogenous taxa and their pollen morphology.
The main aim of this paper is to describe the variation in pollen morphology and to investigate whether this is a useful tool to separate between pollen from different Norwegian hybrids of Sorbus . This will make it possible to study the past distributions of Sorbushybrids.